Synthesis of 1,3-difunctionalized cyclopentenes and 1,3,5-trifunctionalized cyclohexanes by silicon-induced domino reactions

Article abstract of DOI:10.1002/ejoc.200390160

A novel domino process based on a 1,4-C→O shift of a silyl group (4 → 3) and a Michael-induced ring-closure reaction (3 → 2) is investigated. Specifically, attack of carbanions 5 on vinyloxiranes 6 usually occurs on the oxirane unit to give the desired si

Full text of DOI:10.1002/ejoc.200390160

FULL PAPER
Synthesis of 1,3-Difunctionalized Cyclopentenes and 1,3,5-Trifunctionalized
Cyclohexanes by Silicon-Induced Domino Reactions
Frank Tries[a] and Ernst Schaumann*[a]
Dedicated to Professor Lutz F. Tietze on the occasion of his 60th birthday
Keywords: Carbocycles / Domino reactions / Michael-type additions / Oxiranes / Silicon-stabilized carbanions
A novel domino process based on a 1,4-CǞO shift of a silyl
group (4 Ǟ 3) and a Michael-induced ring-closure reaction
unit is required to yield cyclopentenes 1a−d. Analogously,
carbanion 5a and allyloxiranes 15 give cyclization products
(3 Ǟ 2) is investigated. Specifically, attack of carbanions 5 on 19−22, particularly if the ring-closure step is supported by
vinyloxiranes 6 usually occurs on the oxirane unit to give the
desired silyl shift. When starting from vinyloxiranes 6a and
6b, however, the reaction stops at this stage to give silyl
ethers 7. Sulfur (6c) or silicon activation (6d−f) of the C=C
silicon substitution.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2003)
Introduction
The reaction between silyl-stabilized carbanions and
(ω,ωϪ1)-epoxyalkyl tosylates turned out to provide an eleg-
ant domino synthesis[1] of cyclobutanes,[2] cyclopentanes,
and larger carbocycles (Scheme 1).[3] Similarly, bis(epox-
ides) can be employed as bis(electrophiles)[4] and ω-bro-
moalkyl isocyanates afford N-heterocycles.[5] This shows
that both the exo-tet and endo-tet ring-closure modes are
possible in the final cyclization step. We now report on a
novel extension of the domino process in which the crucial
cyclization step obeys the 5-endo-trig mode, as shown for
Scheme 2
the five-membered ring as target in Scheme 2. By analogy
with the chemistry in Scheme 1, the idea was to use silylated
thioacetals 5 as masked dianions, but to employ vinyl-
oxiranes 6 as bis(electrophiles). Oxirane ring-opening to
give 4 should be followed by 1,4-CǞO silicon migration[6]
to yield silyl ether 3. The subsequent cyclization step, an
intramolecular Michael-type addition, should provide
cyclopentane 2, which may be regarded as a formal inter-
mediate of a Peterson olefination starting from a 5-silyl-
alkanal.[7] Consequently, cyclopentenes 1 should be the
final products. However, this reasoning is hampered by in-
herent questions. Is it possible to control the chemoselectiv-
ity of primary attack according to pathway A through a
suitable choice of substituent R2 (Scheme 2)? If so, will the
Scheme 1
[a]
Institut für Organische Chemie, Technischen Universität
Clausthal,
Leibnizstraße 6, 38678 Clausthal-Zellerfeld, Germany
Fax: (internat.) ϩ 49-5323/722858
E-mail: ernst.schaumann@tu-clausthal.de
Eur. J. Org. Chem. 2003, 1085Ϫ1090  2003 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 1434Ϫ193X/03/0306Ϫ1085 $ 20.00ϩ.50/0
1085

F. Tries, E. Schaumann
FULL PAPER
cyclization step 3 Ǟ 2 be possible, as it is generally consid-
The sulfur-substituted oxirane 6c turned out to show
ered a disfavored process for five-membered ring systems?[8] moderate chemoselectivity in the reaction with carbanion
However, examples have been reported for a 5-endo-trig 5a (Scheme 4). The main product is alkenol 11, obviously
process occurring under surprisingly mild conditions.[8c,9]
the result of primary attack on 6c as in pathway B (cf.
Scheme 2) and subsequent opening of the oxirane ring in
the intermediate Michael-type adduct. However, the second
product, cyclopentene 1a, originates from the anticipated
novel domino process and confirms the feasibility of the
sequence of oxirane opening, silicon migration, Michael-
induced ring-closure, and final Peterson olefination (cf.
Scheme 2). As with oxiranes 6a and 6b as starting materials,
a certain reluctance to undergo the final cyclization step
can be seen in the detection of small amounts of silyl ether
10 in some experiments.
Use of Non-Activated Vinyloxiranes 6a and 6b
The desired chemoselectivity (pathway A in Scheme 2)
should be secured if the olefinic bond in 6 is not activated
toward nucleophilic attack, as in the parent vinyloxirane 6a
and its homologue 6b. Actually, oxiranes 6a and 6b undergo
smooth reactions with carbanions 5a and 5b. Careful hydro-
lytic workup affords a product mixture in which silyl ether
7 predominates. This is the protonated form of intermediate
3; the presence of 3 is also documented by isolation of bis-
(adduct) 8 and its desilylated congener 9, the result of addi-
tion of a second equivalent of oxirane 6 to carbanion 3. It
can therefore be concluded that the reaction course follows
the desired pathway A (Scheme 2): exclusive attack of 5 on
the oxirane unit in 6. Moreover, by analogy with the chem-
istry in Scheme 1, the resulting intermediate 2 undergoes
the 1,4-CǞO silyl shift to silyl ether 3. However, there is no
trace of a product that could be derived from cyclization of
3 to a cyclopentene 2. At this point it is an open question
whether the failure of 3 with R2 ϭ H, Me to cyclize is due
to a lack of activation of the CϭC bond for an intramolecu-
lar Michael-type addition or to the inherent problem of a
5-endo-trig process. Further vinyloxiranes 6 with activating
groups therefore had to be investigated.
Scheme 4
In contrast to sulfur derivative 6c, the presence of a sil-
icon substituent R2 Ϫ as in 6dϪf Ϫ produces a complete
shift of the reaction to pathway A (cf. Scheme 2). Cyclo-
pentenes 1 are isolated in all cases; only for TMS derivative
6d is the activation of the CϭC unit for intramolecular nu-
Use of Sulfur- or Silicon-Activated Vinyloxiranes
6c؊f
Because of the required chemoselective attack on the oxi- cleophilic attack so moderate that silyl ether 12 is isolated
rane unit in 6, strongly electron-withdrawing groups R2 (cf. formation of 7 in Scheme 3). Thus, phenyl-substituted
have to be excluded. However, vinyloxiranes 6cϪf with silyl groups R2 offer the perfect solution for cyclopentene
phenylthio or silyl substituents in the R2 position have re- synthesis in the domino process (Scheme 5).
cently become accessible[10] and appear to be ideally suited
as building blocks in the novel domino process (Scheme 2).
Scheme 5
Use of Allyloxiranes 15 To Give Cyclohexanes
19؊22
The domino process should be amenable to the synthesis
of other functionalized carbocycles. This was demonstrated
Scheme 3
1086
by the extension of the approach to cyclohexane synthesis
Eur. J. Org. Chem. 2003, 1085Ϫ1090

1,3-Difunctionalized Cyclopentenes and 1,3,5-Trifunctionalized Cyclohexanes
FULL PAPER
was distilled from CaH2. Column chromatography: Merck silica gel
(70Ϫ230 mesh). Petroleum ether (PE), boiling range 60Ϫ70 °C, was
always used in the separations. Analytical TLC: Merck silica gel 60
PF254 plates (visualization with UV light or 4-methoxybenzal-
dehyde spray reagent). Melting points are uncorrected. Elemental
analyses: Institut für Pharmazeutische Chemie, TU Braunschweig.
Protonated 5a, 5b[12] vinyloxiranes 6b,[13] 6cϪf,[10] and (1-bromovi-
nyl)(triphenyl)silane (13b)[14] were prepared according to literature
procedures. Vinyloxirane (6a) and vinyl sulfide 13a were commer-
cially available.
Synthesis of Carbocycles by the Silicon-Induced Domino Reaction.
General Procedure: A solution of dithioacetal 5 (1.2 equiv.) in dry
THF (3 mL/mmol) was cooled to Ϫ78 °C. nBuLi (1.6  solution
in hexane, 1.1 equiv.) and TMEDA (1.2 equiv.) were then added,
and the solution was allowed to warm to 0 °C over 2 h. This tem-
perature was maintained for 1 h, and the mixture was again cooled
to Ϫ78 °C. To this mixture, 6 or 15 (1.0 equiv.) in dry THF (20 mL/
mmol) was added dropwise. The yellow reaction mixture was al-
lowed to warm to room temperature over 12 h. For workup, it was
hydrolyzed with a mixture of diethyl ether/saturated NH4Cl solu-
tion (1:1). The aqueous phase was separated and extracted twice
with diethyl ether. The combined organic layers were washed with
brine, dried (MgSO4), and concentrated in vacuo.
Scheme 6
(Scheme 6). The required starting materials are allyl-
oxiranes 15, which can be obtained by a modification of
a published procedure.[11] The key step is an alkylation of
lithiated alkenes 13 by epichlorohydrin. In the silicon case,
this produces a mixture of chloroalkanol 14b and oxirane
15b, but complete conversion into the oxirane can be
achieved by treatment with potassium carbonate.
Reaction of Dithioacetal 5a with Vinyloxirane (6a): Protonated 5a
(260 mg, 1.2 mmol) and vinyloxirane (6a; 70 mg, 1.0 mmol) gave
three fractions after purification by flash chromatography (eluent
PE Ǟ PE/ethyl acetate, 20:1 Ǟ 10:1 Ǟ 5:1 Ǟ 1:1).
First Fraction. 5,5-Bis(methylthio)-3-(trimethylsiloxy)pent-1-ene
(7a): Yield 92 mg (37%), yellow oil. IR (film): ν˜ ϭ 1643 (CϭC),
1423, 1251, 842 (SiMe3), 1083, 1027, 926, 753 cmϪ1
.
1H NMR
The silicon-substituted allyloxirane 15b gives the desired
domino process to afford functionalized cyclohexane 19b.
Thus, by analogy with cyclopentene synthesis (Scheme 2),
the reaction with anion 5a is initiated by chemoselective
oxirane ring-opening to give 16b, followed in situ by silyl
migration to 17b and intramolecular Michael addition to
give 18b. Workup gives cyclohexane 19b, along with desilyl-
ated material 20b; interestingly, partial loss of methanethiol
is also observed and confirmed by isolation of cyclohexene
22. Here, the regioselectivity of thiol elimination was deter-
mined by means of NMR experiments (cf. Exp. Sect.).
3
3
(200 MHz): δ ϭ 0.12 (s, 9 H, SiMe3), 1.80 (dd, J ϭ 4.6, J ϭ 9.4,
1 H, CH2), 1.91 (dd, 3J ϭ 5.4, 3J ϭ 8.6, 1 H, 4-CH2), 2.08 (s, 3 H,
SMe), 2.09 (s, 3 H, SMe), 3.73 [dd, 3J ϭ 5.4, 3J ϭ 9.4, 1 H,
CH(SMe)2], 4.41 (m, 1 H, CHOSi), 5.06 (dt, 2J ϭ 1.2, 4J ϭ 1.2,
2
4
3
3J ϭ 10.4, 1 H, olef. CH2), 5.19 (dt, J ϭ 1.2, J ϭ 1.2, J ϭ 17.2,
1 H, olef. CH2), 5.79 (ddd, 3J ϭ 6.4, 3J ϭ 10.4, 3J ϭ 17.2, 1 H,
olef. CH) ppm. 13C NMR (50 MHz): δ ϭ 140.8 (olef. CH), 114.6
(olef. CH2), 71.3 (CHOSi), 42.7 (4-CH2), 50.5 [CH(SMe)2], 12.5,
12.1 (2 SMe), 0.3 (SiMe3) ppm.
Second Fraction. 5,5-Bis(methylthio)-7-(trimethylsiloxy)nona-1,8-di-
enol (8a): Yield 35 mg (14%), yellow oil. IR (film): ν˜ ϭ 3448 (OH),
The reaction between the sulfur-substituted allyloxirane 1643 (CϭC), 1422, 1252, 1083, 923, 842 (SiMe3), 753, 676 cmϪ1
.
1H NMR (200 MHz): δ ϭ 0.1 (s, 9 H, SiMe3), 1.75 (m, 1 H, CH2),
15a and carbanion 5a gives a complex reaction mixture in
which cyclohexane 19a is probably present, but only elim-
ination product 21 could be characterized. This result un-
derlines the greater utility of the silicon-activated oxiranes
also in cyclohexane synthesis, including the option of the
use of optically active epichlorohydrins to give enantiopure
19b and congeners.
3
2
1.86 (dd, J ϭ 9.6, J ϭ 15.2, 1 H, CH2), 1.95 (s, 3 H, SMe), 2.10
3
2
(s, 3 H, SMe), 2.25 (dd, J ϭ 8.8, J ϭ 15.2, 2 H, CH2), 4.02 (d,
2J ϭ 1.6, 1 H, OH), 4.65 (m, 2 H, CHOH, CHOSi), 5.04 (dd, J ϭ
4
3
4
3
1.2, J ϭ 10.0, 1 H, olef. CH2), 5.08 (dd, J ϭ 1.2, J ϭ 9.2, 1 H,
4
3
olef. CH2), 5.15 (dt, J ϭ 1.2, J ϭ 17.2, 1 H, olef. CH2), 5.29 (dt,
4J ϭ 1.6, 3J ϭ 17.2, 1 H, olef. CH2), 5.82 (ddd, J ϭ 5.2, 3J ϭ 10.0,
J ϭ 16.8, 1 H, olef. CH), 5.87 (ddd, J ϭ 7.2, J ϭ 10.2, J ϭ 17.2,
1 H, olef. CH) ppm. 13C NMR (50 MHz): δ ϭ 142.0, 140.1 (olef.
CH), 114.3, 114.1 (olef. CH2), 72.3, 69.9 (CHO), 61.7 [C(SMe)2],
42.7, 41.5 (CH2), 11.9, 10.7 (SMe), 0.7 (SiMe3) ppm.
Experimental Section
General: NMR: Bruker DPX 200 (200 MHz) or ARX 400
(400 MHz); solvent CDCl3 unless stated otherwise, with TMS as
internal standard; coupling constants J are given in Hz; assign-
ments of 13C NMR signals were supported by broadband-de-
coupled DEPT. IR: Bruker Vektor 22 or Pye-Unicam SP3-200
spectrometers. All experiments were performed under dry N2. THF
was distilled from sodium benzophenone ketyl prior to use. CH2Cl2
Third Fraction. 5,5-Bis(methylthio)nona-1,8-diene-3,7-diol (9a):
Yield 26 mg (10%), yellow oil. IR (film): ν˜ ϭ 3281 (OH), 1644 (Cϭ
1
C), 1423, 993, 922, 733 cmϪ1. H NMR (200 MHz): δ ϭ 1.82 (d,
3
2J ϭ 15.6, 2 H, 2 CH2), 2.01 (s, 6 H, 2 SMe) 2.27 (dd, J ϭ 9.4,
2J ϭ 15.6, 2 H, 2 CH2), 3.76 (s, 2 H, 2OH), 4.60 (m, 2 H, 2 CHOH),
5.08 (dt, 2J ϭ 1.2, 4J ϭ 1.2, 3J ϭ 10.4, 2 H, 2 olef. CH2), 5.86 (ddd,
3
3
3J ϭ 6.0, J ϭ 10.4, J ϭ 16.8, 2 H, 2 olef. CH2) ppm. 13C NMR
Eur. J. Org. Chem. 2003, 1085Ϫ1090
1087

F. Tries, E. Schaumann
1H NMR (200 MHz,
FULL PAPER
(50 MHz): δ ϭ 141.0 (2 olef. CH), 114.3 (2 olef. CH2), 70.0 (2 C), 1477, 1436, 1250, 1092, 746, 693 cmϪ1
.
CHOH), 62.1 [C(SMe)2], 41.9 (2 CH2), 11.2 (2 SMe) ppm.
CDCl3): δ ϭ 1.78 (s, 6 H, SMe), 2.59, 2.60, 2.80, 2.81 (m, 1 H each,
2 CH2), 5.56 (m, 1 H, olef. CH), 6.90Ϫ7.06 (m, 3 H, SPh),
7.40Ϫ7.48 (m, 2 H, SPh) ppm. 13C NMR (50 MHz, CDCl3): δ ϭ
134.4, 133.7 (arom. C, olef. C), 131.1, 130.1, 129.3, 128.3, 127.1
(olef. CH, arom.CH), 63.1 (CSMe), 50.3, 48.4 (2 CH2), 12.9 (2
SMe) ppm.
2-[2-(Trimethylsiloxy)but-3-enyl]-1,3-dithiane (7b): Yield 61 mg
(23%) from the reaction between protonated 5b (230 mg, 1.2 mmol)
and oxirane 6a (70 mg, 1.0 mmol) after purification by flash chro-
matography (eluent PE Ǟ PE/ethyl acetate stepwise to 1:1), yellow
oil. IR (film): ν˜ ϭ 1644 (CϭC), 1422, 1251, 1128, 1082, 1027, 994,
1
931, 883, 843 (SiMe3), 754 cmϪ1. H NMR (200 MHz): δ ϭ 0.13
Second Fraction. 5,5-Bis(methylthio)-3-(phenylthio)-5-(trimethylsi-
lyl)pent-2-en-1-ol (11): 140 mg (78%), yellow oil, isomer mixture
(3.6:1*). Assignment to the cis and trans forms was not possible.
IR (film): ν˜ ϭ 3365 (OH), 1623, 1582 (CϭC), 1248, 843 (SiMe3),
(s, 9 H, SiMe3), 1.80Ϫ1.92 (m, 2 H, CH2), 2.72Ϫ2.90 [m, 6 H,
S(CH2)3S], 4.04 (dd, 3J ϭ 6.0, 3J ϭ 8.6, 1 H, CHS), 4.38 (m,
2
4
3
CHOSi), 5.06 (dt, J ϭ J ϭ 1.4, J ϭ 10.4, 1 H, olef. CH2), 5.20
(dt, 2J ϭ J ϭ 1.4, 3J ϭ 17.2, 1 H, olef. CH2), 5.79 (ddd, 3J ϭ 6.2,
1
4
1022, 744, 693 (Ph) cmϪ1. H NMR (200 MHz, CDCl3): δ ϭ 0.17
3J ϭ 10.4, 3J ϭ 17.2, 1 H, olef. CH) ppm. 13C NMR (50 MHz):
δ ϭ 140.6 (olef. CH), 114.6 (olef. CH2), 70.1 (CHOSi), 43.5 (CHS),
43.2 (CH2), 31.0, 30.2, 29.8 [S(CH2)3S] ppm.
(s, 9 H, SiMe3), 0.22* (s, 9 H, SiMe3*), 2.03 (s, 6 H, SMe), 2.17*
(s, 6 H, SMe*), 2.72 (s, 2 H, CH2), 2.84* (s, 2 H, CH2*), 4.16* (d,
3
3J ϭ 7.8*, 2 H, CH2OH), 4.38 (d, J ϭ 5.8, 2 H, CH2OH), 5.62*
(t, 3J ϭ 7.8, 1 H, olef. CH), 6.43 (t, 3J ϭ 5.8, 1 H, olef. CH),
7.13Ϫ7.45 (m, 5 H, arom. CH) ppm. 13C NMR (50 MHz, CDCl3):
δ ϭ 140.9, 133.4, 133.3*, 129.3*, 129.1, 128.1*, 126.2 (arom. CH,
olef. CH), 137.8*, 135.6, 131.3, 130.8* (arom. C, olef. C), 61.2,
59.0* (CH2OH), 47.6, 45.8* (CSMe), 43.5, 39.0* (CH2), 12.9*, 12.0
(SMe), Ϫ0.9, Ϫ1.5* (SiMe3) ppm. C16H26OS3Si (358.6): calcd. C
53.62, H 7.32, S 26.79; found C 54.92, H 7.65, S 26.59.
Reaction between Thioacetal 5a and Oxirane 6b: Protonated 5a
(216 mg, 1.2 mmol) and 6b (84 mg, 1.0 mmol) reacted to give three
fractions after flash chromatography (eluent PE Ǟ PE/ethyl acet-
ate, 40:1 Ǟ 20:1 Ǟ 10:1 Ǟ 5:1 Ǟ 1:1).
First Fraction. 2-Methyl-5,5-bis(methylthio)-3-(trimethylsiloxy)pent-
1-ene (7c): Yield 55 mg (21%), yellow oil. IR (film): ν˜ ϭ 1650 (Cϭ
C), 1436, 1252, 842 (SiMe3), 1087, 751 cmϪ1. 1H NMR (200 MHz):
3
Independent Experiment. 5,5-Bis(methylthio)-2-(phenylthio)-3-(tri-
methylsiloxy)pent-1-ene (10): Yield 14 mg (9%), yellow oil. IR
(film): ν˜ ϭ 1607, 1582 (CϭC), 1477, 1436, 1250, 1092, 844 (SiMe3),
δ ϭ 0.11 (s, 9 H, SiMe3), 1.69 (s, 3 H, CCH3), 1.81 (dd, J ϭ 4.4,
3J ϭ 9.4, 1 H, CH2), 1.92 (dd, 3J ϭ 5.4, 3J ϭ 8.8, 1 H, CH2), 2.07,
2.10 (s, 3H each, SMe), 3.71 [dd, 3J ϭ 5.4, 3J ϭ 9.4, 1 H,
746, 693 cmϪ1 1H NMR (200 MHz, CDCl3): δ ϭ 0.08 (s, 9 H,
.
3
3
CH(SMe)2], 4.39 (dd, J ϭ 4.4, J ϭ 8.8, 1 H, CHOSi), 4.80, 4.93
(each d, J ϭ 1.4, 1 H, olef. CH2) ppm. 13C NMR (50 MHz): δ ϭ
147.2 (olef. C), 111.3 (olef. CH2), 74.0 (CHOSi), 50.9 [CH(SMe)2],
41.2 (CH2), 17.1 (CH3), 12.8, 11.9 (SMe), 0.1 (SiMe3) ppm.
SiMe3), 1.94Ϫ2.08 (m, 2 H, CH2), 1.99, 2.06 (each s, 3 H, SMe),
3
3
3
3.71 [dd, J ϭ 5.0, J ϭ 10.0, 1 H, CH(SMe)2], 4.47 (dd, J ϭ 3.6,
3J ϭ 8.8, 1 H, CHOSi), 4.82 (s, 1 H, olef. CH2), 5.40 (d, J ϭ 0.8 Hz,
1 H, olef. CH2), 7.15Ϫ7.45 (m, 5 H, arom. CH) ppm. 13C NMR
(50 MHz, CDCl3): δ ϭ 149.3 (olef. C), 132.4 (arom. C), 129.3,
129.2, 127.1 (arom. CH), 116.4 (olef. CH2), 72.6 (CHOSi), 50.9
(CHS), 42.3 (CH2), 13.1, (SMe), 0.1 (SiMe3) ppm.
Second Fraction. 2,8-Dimethyl-5,5-bis(methylthio)-7-(trimethyl-
siloxy)nona-1,8-dien-3-ol (8b): Yield 76 mg (22%), yellow oil. IR
(film): ν˜ ϭ 3447 (OH), 1650 (CϭC), 1438, 1372, 1252, 1091, 1010,
1
972, 947, 901, 842 (SiMe3), 753, 689 cmϪ1. H NMR (200 MHz):
δ ϭ 0.07 (s, 9 H, SiMe3), 1.66Ϫ1.78 (m, 1 H, CH2), 1.72 (s, 3 H,
CCH3), 1.77 (s, 3 H, CCH3), 1.95Ϫ2.01 (m, 1 H, CH2), 1.98 (s, 3
Reaction between Thioacetal 5a and Oxirane 6d: Compound 6d
(71 mg, 0.5 mmol) and protonated 5a (108 mg, 0.6 mmol) gave a
yellow oil (90 mg) as a mixture of 1b (23%) and 12 (41%) after
flash chromatography (eluent PE/EE, 5:1) and circular chromato-
graphy (chromatotron).
3
2
H, SMe), 2.11 (dd, J ϭ 2.0, J ϭ 15.2, 1 H, CH2), 2.12 (s, 3 H,
SMe), 2.30 (dd, 3J ϭ 10.0, 2J ϭ 15.2, 1 H, CH2), 3.90 (d, 3J ϭ 1.6,
2
4
1 H, OH), 4.60 (m, 2 H, CHOH, CHOSi), 4.77 (t, J ϭ J ϭ 1.6,
1 H, olef. CH2), 4.82 (t, J ϭ 1.6 Hz, 1 H, olef. CH2), 4.92 (t, J ϭ
1.0 Hz, 1 H, olef. CH2), 4.98 (t, J ϭ 1.0 Hz, 1 H, olef. CH2) ppm.
13C NMR (50 MHz): δ ϭ 148.6, 146.8 (olef. C), 111.3, 111.2 (olef.
CH2), 74.7, 73.0 (CHOH, CHOSi), 61.8 [C(SMe)2], 41.8, 40.5
(CH2), 18.0, 17.1 (CCH3), 12.0, 10.9 (SMe), 0.3 (SiMe3) ppm.
4,4-Bis(methylthio)-1-(trimethylsilyl)cyclopent-1-ene (1b): IR (film):
ν˜ ϭ 1435, 1422, 1249 (SiMe3), 1081, 933, 839 (SiMe3), 756, 691
cmϪ1. 1H NMR (200 MHz, CDCl3): δ ϭ 0.19 (s, 9 H, SiMe3), 1.43
3
(s, 4 H, CH2), 2.03 (s, 6 H, SMe), 6.71 (t, J ϭ 6.4, 1 H, olef. CH)
ppm. 13C NMR (50 MHz, CDCl3): δ ϭ 144.9 (olef. CH), 135.7
(olef. C), 42.2, 40.2 (CH2), 12.9, 11.7 (SMe), 0.8 (SiMe3) ppm. GC-
MS: m/z ϭ 232 (0.5) [Mϩ], 211 (12), 185 (11) [M Ϫ SMe], 106 (10),
105 (26), 91 (43), 73 (SiMe3, 100), 59 (10).
Third Fraction: 2,8-Dimethyl-5,5-bis(methylthio)nona-1,8-diene-3,7-
diol (9b): Yield 40 mg (15%), yellow oil. IR (film): ν˜ ϭ 3281 (OH),
1
1644 (CϭC), 1423, 993, 922, 733 cmϪ1. H NMR (200 MHz): δ ϭ
2
1.74 (s, 6 H, 2 CCH3), 1.77 (broad d, J ϭ 15.8, 1H of each CH2),
2.0 (s, 6 H, 2 SMe), 2.31 (dd, 3J ϭ 9.6, 2J ϭ 15.8, 1 H of each
5,5-Bis(methylthio)-3-(trimethylsiloxy)-2-(trimethylsilyl)pent-1-ene
CH2), 3.90 (s, 2 H, 2OH), 4.57 (broad d, 3J ϭ 9.6, 2 H, 2 CHOH),
(12): IR (film): ν˜ ϭ 1435, 1422, 1249 (SiMe3), 1081, 933, 839
2
4
4.79 (t, J ϭ J ϭ 1.4, 1 H of each olef. CH2), 4.97 (broad s, 1 H
of each olef. CH2) ppm. 13C NMR (50 MHz): δ ϭ 147.9 (olef. C),
110.8 (olef. CH2), 72.7 (CHOH), 62.9 [o, C(SMe)2], 40.7 (CH2),
17.7 (CCH3), 11.2 (SMe) ppm.
(SiMe3), 756, 691 cmϪ1 1H NMR (200 MHz, CDCl3): δ ϭ 0.09,
.
3
3
0.13 (each s, 9 H, SiMe3), 1.79 (dd, J ϭ 4.4, J ϭ 9.2, 1 H, CH2),
1.82 (dd, 3J ϭ 5.2, J ϭ 8.6, 1 H, CH2), 2.08, 2.09 (each s, 3 H,
3
SMe), 3.75 [dd, 3J ϭ 5.2, 3J ϭ 9.2, 1 H, CH(SMe)2], 4.57 (ddt,
4J ϭ 1.0, 3J ϭ 4.4, 3J ϭ 8.6, 1 H, CHOSi), 5.38 (dd, 4J ϭ 1.0, 2J ϭ
Reaction between Thioacetal 5a and Oxirane 6c: The reaction be-
tween protonated 5a (108 mg, 0.6 mmol) and 6c (89 mg, 0.5 mmol)
gave two fractions after flash chromatography (eluent PE Ǟ PE/
ethyl acetate, 40:1 Ǟ 5:1).
4
2
3.0, 1 H, olef. CH2), 5.82 (dd, J ϭ 1.2, J ϭ 3.0, 1 H, olef. CH2)
ppm. 13C NMR (50 MHz, CDCl3): δ ϭ 154.6 (olef. C), 124.5 (olef.
CH2), 74.2 (CHOSi), 51.1 [CH(SMe)2], 43.9 (CH2), 12.6, 12.2
(SMe), Ϫ0.3, Ϫ0.9 (SiMe3) ppm. GC-MS: m/z ϭ 322 (3) [M], 249
(3) [M Ϫ SiMe3], 248 (11), 233 (5) [M Ϫ OSiMe3], 202 (19), 147
First Fraction: 4,4-Bis(methylthio)-1-(phenylthio)cyclopent-1-ene
(1a): Yield 30 mg (22%), yellow oil. IR (film): ν˜ ϭ 1607, 1582 (Cϭ (35), 113 (23), 107 (21), 74 (13), 73 (100) [SiMe3], 59 (13).
1088 Eur. J. Org. Chem. 2003, 1085Ϫ1090

1,3-Difunctionalized Cyclopentenes and 1,3,5-Trifunctionalized Cyclohexanes
FULL PAPER
1-[Dimethyl(phenyl)silyl]-4,4-bis(methylthio)cyclopent-1-ene
(1c): C), 1484, 1428, 1260, 1188, 1110, 973, 944, 908, 850, 831, 740, 702
1
3
This compound was prepared from 6e (98 mg, 0.48 mmol) and pro-
tonated 5c (104 mg, 0.58 mmol), purification by flash chromato-
graphy (PE/ethyl acetate, 5:1) and circular chromatography
(chromatotron, eluent PE). Yield 92 mg (65%), light yellow oil. IR
cmϪ1 (Ph). H NMR (200 MHz, CDCl3): δ ϭ 2.19 (dd, J ϭ 2.8,
4
3
2
2J ϭ 5.0, 1 H, oxirane CH2), 2.27 (ddt, J ϭ 1.2, J ϭ 5.8, J ϭ
16.0, 1 H, allylic CH2), 2.47 (m, 1 H, allylic CH2), 2.53 (m, 1 H,
oxirane CH2), 2.84 (tdd, 3J ϭ 2.8, 3J ϭ 4.0, 3J ϭ 5.8, 1 H, oxirane
4
2
4
(film): ν˜ ϭ 1597 (CϭC), 1427, 1252 (SiMe2), 1198, 1113 (CϪO), CH), 5.60 (dt, J ϭ 1.2, J ϭ 2.4, 1 H, olef. CH2), 6.13 (dt, J ϭ
1039, 846 (SiMe2), 830, 799, 773, 731, 700 (Ph) cmϪ1
.
1H NMR
1.6, 2J ϭ 2.4, 1 H, olef. CH2), 7.24Ϫ7.40 (m, 9 H, arom. CH),
7.46Ϫ7.53 (m, 6 H, arom. CH) ppm. 13C NMR (50 MHz, CDCl3):
(200 MHz, CDCl3): δ ϭ 0.38 (s, 6 H, SiMe2), 2.08 (s, 6 H, SMe),
2.80 (m, 2 H, CH2), 2.86 (m, 2 H, CH2), 6.00 (t, J ϭ 2.0 Hz, 1 H, δ ϭ 142.4 (olef. C), 136.2, 129.7, 127.9 (arom. CH), 133.4 (arom.
olef. CH), 7.33Ϫ7.40 (m, 3 H, arom. CH), 7.48Ϫ7.57 (m, 2 H, C), 132.8 (olef. CH2), 51.5 (oxirane CH), 47.2 (oxirane CH2), 39.0
arom. CH) ppm. 13C NMR (50 MHz, CDCl3): δ ϭ 140.9, 137.7
(CH2) ppm. C23H22OSi (342.14): calcd. C 80.67, H 6.48; found C
(arom. C, olef. C), 139.3, 133.8, 129.0, 127.8 (arom. CH, olef. CH), 80.12, H 6.35.
133.8 (arom. C), 63.6 [C(SMe)2], 51.3, 50.2 (CH2), 13.4 (SMe),
Ϫ3.2 (SiMe2) ppm. C15H22S2Si (294.1): calcd. C 61.21, H 7.54, S
21.74; found C 60.92, H 7.84, S 20.65.
Second Fraction. 1-Chloro-4-(triphenylsilyl)pent-4-en-2-ol (14b):
Yield 465 mg (16%), colorless oil. IR (film): ν˜ ϭ 3569, 3561, 3433
(OH), 1589 (CϭC), 1428, 1109, 743, 704 cmϪ1 (Ph). 1H NMR
(200 MHz, CDCl3): δ ϭ 1.98 (d, 3J ϭ 4.6, 1 H, OH), 2.48 (ddd,
4J ϭ 1.2, 3J ϭ 7.2, 2J ϭ 14.4, allylic CH2), 2.58 (ddd, 4J ϭ 1.2,
3J ϭ 5.4, 2J ϭ 14.4, 1 H, allylic CH2), 3.34 (dd, 3J ϭ 6.0, 2J ϭ
11.2, 1 H, CH2Cl), 3.45 (dd, 3J ϭ 4.0, 2J ϭ 11.2, 1 H, CH2Cl),
4,4-Bis(methylthio)-1-(triphenylsilyl)cyclopent-1-ene (1d): This com-
pound was prepared from 6f (82 mg, 0.25 mmol) and 5a (34 mg,
0.3 mmol); flash chromatography (PE/ ethyl acetate, 5:1) and circu-
lar chromatography (chromatotron, eluent PE). Yield 63 mg (61%)
light brown crystals (m.p. 110Ϫ111 °C). IR (film): ν˜ ϭ 1591 (Cϭ
C), 1428, 1111, 909, 737, 702 cmϪ1 (Ph). 1H NMR (200 MHz,
CDCl3): δ ϭ 2.08 (s, 6 H, SMe), 2.89, 2.96 (each m, 2 H, CH2),
2
3.62 (m, 1 H, CHOH), 5.75 (d, J ϭ 2.6, 1 H, olef. CH2), 6.16 (dt,
4J ϭ 1.2, 2J ϭ 2.6, 1 H, olef. CH2), 7.34Ϫ7.49 (m, 9 H, arom.CH),
7.55Ϫ7.62 (m, 6 H, arom. CH) ppm. 13C NMR (50 MHz, CDCl3):
δ ϭ 142.5 (olef. C), 136.2, 129.8, 128.0 (arom.CH), 133.0 (arom.
C), 134.2 (olef. CH2), 69.7 (HCOH), 49.5 (CH2Cl), 41.5 (allylic
CH2) ppm.
3
3
6.09 (dd, J ϭ 2.0, J ϭ 2.0, 1 H, olef. CH), 7.36Ϫ7.45 (m, 9 H,
arom. CH), 7.50Ϫ7.58 (m, 6 H, arom. CH) ppm. 13C NMR
(50 MHz, CDCl3): δ ϭ 144.5 (olef. CH), 137.7 (olef. C), 136.0,
129.6, 127.9 (arom. CH), 133.8 (arom. C), 64.2 [C(SMe)2], 51.7,
50.2 (CH2), 13.4 (SMe) ppm. C25H26S2Si (418.1): calcd. C 71.75,
H 6.27, S 15.29; found C 71.89, H 6.30, S 14.73.
Conversion of Chlorohydrin 14b into Oxirane 15b: K2CO3 (80 mg,
0.58 mmol) was added to 14b (178 mg, 0.47 mmol) in methanol
(2.5 mL). The solution was stirred for several minutes and the reac-
tion was monitored by TLC. If necessary, further K2CO3 was ad-
ded. For workup, diethyl ether (45 mL/mmol) was added and the
reaction mixture was filtered. The organic phase was washed with
saturated NH4Cl solution, water, and brine, dried (MgSO4), and
concentrated in vacuo. Flash chromatography (PE/ethyl acetate,
60:1) gave 15b (123 mg, 76%) as colorless crystals (m.p. 75Ϫ76 °C).
General Procedure for the Preparation of Allyloxiranes 15b from Al-
kenes 13: An acceptor-substituted α-lithioethene (1.0 equiv.) in
THF (2 mL/mmol) was treated dropwise at Ϫ78 °C (Ϫ30 °C for
the sulfur compound) with epichlorohydrin (1.0 equiv.), dissolved
in THF (3 mL/mmol). The reaction mixture was allowed to warm
to room temperature over 12 h. For workup, it was hydrolyzed with
a mixture of diethyl ether/saturated NH4Cl solution (1:1). The
aqueous phase was separated, extracted twice with diethyl ether,
and washed with brine. The combined organic layers were washed
with brine, dried (MgSO4), and concentrated in vacuo.
3-(Methylthio)-5-(phenylthio)cyclohex-2-en-1-ol (21): Preparation
from 5a (216 mg, 1.2 mmol) and 15a (192 mg 1.0 mmol); purifica-
tion by flash chromatography (eluent PE Ǟ PE/ethyl acetate, 120:1
Ǟ 5:1). Yield 98 mg (39%) as a not quite pure solid. IR (film): ν˜ ϭ
3397 (OH), 1623, 1582 (CϭC), 1477, 1438, 1068, 1025, 747, 692
Reaction between Vinyl Sulfide 13a and Epichlorohydrin: Com-
pound 13a (1.362 g, 10.0 mmol) and epichlorohydrin (925 mg,
10.0 mmol) gave 648 mg (48%) recovered 13a and 15a after flash
chromatography (eluent PE Ǟ PE/ethyl acetate, 40:1 Ǟ 10:1.
3
cmϪ1 (Ph). 1H NMR (200 MHz, CDCl3): δ ϭ 1.64 (ddd, J ϭ 8.4,
2
3J ϭ 11.2, J ϭ 12.8, 1 H, 6-Hax), 2.23 (s, 3 H, SMe), 2.33 (m, 1
3
3
H, 6-Heq), 2.37 (m, 2 H, 4-CH2), 3.38 (dddd, J ϭ 2.8, J ϭ 5.6,
3J ϭ 9.2, 3J ϭ 11.2, 1 H, 5-Hax), 4.35 (ddd, 3J ϭ 2.4, 3J ϭ 6.0,
3J ϭ 8.4, 1 H, 1-Hax), 5.34 (broad s, 1 H, olef. CH), 7.25Ϫ7.47 (m,
5 H, arom. CH) ppm. 13C NMR (50 MHz, CDCl3): δ ϭ 136.0,
132.2 (olef. C, arom. C), 132.9, 129.0, 127.6 (arom. CH), 119.7
(olef. CH), 67.6 (HCOH), 41.4 (HCSPh), 38.5, 36.2 (CH2), 14.0
(SMe) ppm.
1-(Oxiranylmethyl)vinyl Phenyl Sulfide (15a): Yield 733 mg (38%),
yellow oil. IR (film): ν˜ ϭ 1610 (CϭC), 1583 (CϭC), 1476, 1439,
1
835, 749, 692 cmϪ1 (Ph). H NMR (200 MHz, CDCl3): δ ϭ 2.37
(ddd, 4J ϭ 0.8, 3J ϭ 6.0, 2J ϭ 15.6, 1 H, allylic CH2), 2.50 (dd,
3J ϭ 2.8, 2J ϭ 4.8, 1 H, oxirane CH2), 2.53 (dd, 3J ϭ 6.0, 2J ϭ
3
2
15.6, 1 H, allylic CH2), 2.80 (dd, J ϭ 4.0, J ϭ 4.8, 1 H, oxirane
3
3
3
CH2), 3.14 (ddt, J ϭ 2.8, J ϭ 4.0, J ϭ 6.0, 1 H, oxirane CH),
5.03 (s, 1 H, olef. CH2), 5.40 (broad s, 1 H, olef. CH2), 7.28Ϫ7.48
(m, 5 H, arom. CH) ppm. 13C NMR (50 MHz, CDCl3): δ ϭ 141.2
(olef. C), 133.1, 129.2, 128.0 (arom. CH), 132.5 (arom. C), 115.0
(olef. CH2), 50.9 (oxirane CH), 47.0 (oxirane CH2), 39.6 (allylic
CH2) ppm.
Reaction between Thioacetal 5a and Allyloxirane 15b: Protonated
5a (78 mg, 0.43 mmol) and 15b (103 mg, 0.3 mmol) gave two frac-
tions after flash chromatography (eluents PE Ǟ PE/ethyl acetate,
150:1 Ǟ 5:1).
First Fraction. 3,3-Bis(methylthio)-1-(trimethylsiloxy)-5-(triphenylsi-
lyl)cyclohexane (19b): Yield 55 mg (35%). IR (film): ν˜ ϭ 1589 (Cϭ
C), 1428, 1250 (SiMe3), 1110 (CϪO), 1065, 908, 841 (SiMe3), 741,
702 cmϪ1 (Ph). 1H NMR (200 MHz, CDCl3): δ ϭ 0.08 (s, 9 H,
SiMe3), 1.47Ϫ1.80 (m, 4 H, 2 CH2), 1.94, 2.02 (each s, 3 H, SMe),
2.18Ϫ2.44 (m, 2 H, CH2), 2.35 (tt, J ϭ 2.6, J ϭ 13.2, 1 H, HCSi),
4.05 (tt, J ϭ 4.4, J ϭ 10.8, 1 H, HCOSi), 7.32Ϫ7.61 (m, 15 H,
arom. CH) ppm. 13C NMR (50 MHz, CDCl3): δ ϭ 135.9, 129.6,
128.0 (arom. CH), 133.3 (arom. C), 67.9 (HCOSi), 60.7 [C(SMe)2],
Reaction between Vinylsilane 13b and Epichlorohydrin: Compound
13b (2.812 g, 7.7 mmol) and epichlorohydrin (712 mg, 7.7 mmol)
gave 1.37 g (62%) of recovered 13b and two additional fractions
after flash chromatography (eluent PE Ǟ PE/ethyl acetate, 40:1 Ǟ
20:1 Ǟ 10:1 Ǟ 5:1).
First Fraction. [2-(Triphenylsilyl)allyl]oxirane (15b): Yield 230 mg
(9%), colorless crystals (m.p. 75Ϫ76 °C). IR (film): ν˜ ϭ 1588 (Cϭ
Eur. J. Org. Chem. 2003, 1085Ϫ1090
1089

F. Tries, E. Schaumann
FULL PAPER
[1]
[1a] L. F. Tietze, U. Beifuss, Angew. Chem. 1993, 105, 137Ϫ170;
45.7, 36.5, 36.4 (CH2), 17.9 (HCSi), 11.3, 11.1 (SMe), 1.0 (SiMe3)
ppm.
[1b]
Angew. Chem. Int. Ed. Engl. 1993, 32, 131Ϫ163.
P. J. Par-
sons, C. S. Penkett, A. J. Shell, Chem. Rev. 1996, 96, 195Ϫ206.
Second Fraction: Solid, inseparable mixture of 20b (30%) and 22
(10%). Determination of both structures was based on NOE and
also inverse H,H-COSY and C,H-COSY NMR measurements.
[2] [2a]
N. Bräuer, Diplom thesis, Clausthal, 1994. [2b]T. Takeda,
S. Naito, K. Ando, T. Fujiwara, Bull. Chem. Soc. Jpn. 1983,
56, 976Ϫ978.
T. Michel, A. Kirschning, C. Beier, N. Bräuer, E. Schaumann,
G. Adiwidjaja, Liebigs Ann. 1996, 1811Ϫ1821.
N. Bräuer, S. Dreeßen, E. Schaumann, Tetrahedron Lett. 1999,
40, 2921Ϫ2924.
A. Jung, O. Koch, M. Ries, E. Schaumann, Synlett 2000,
92Ϫ94.
W. H. Moser, Tetrahedron 2001, 57, 2065Ϫ2084.
D. J. Ager, Org. React. 1990, 38, 1Ϫ223.
[3]
[4]
[5]
3,3-Bis(methylthio)-5-(triphenylsilyl)cyclohexanol (20b): IR (film):
ν˜ ϭ 3416 (OH), 1589 (CϭC), 1428, 1109 (CϪO), 1038, 847, 738,
702 cmϪ1 (Ph). 1H NMR (400 MHz, CDCl3): δ ϭ 0.79 (s, 1 H,
OH), 1.29 (td, J ϭ 11.0, J ϭ 12.8, 1 H, 6-Hax), 1.56 (dd, J ϭ 9.0,
J ϭ 13.2, 1 H, 2-Hax), 1.84 (s, 3 H, SMe), 1.85 (t, J ϭ 12.8 Hz, 1
H, 4-Hax), 1.85 (s, 3 H, SMe), 2.31 (tt, J ϭ 6.6, J ϭ 2.4, 1 H, 6-
Heq), 2.35 (m, 2 H, 2-Heq, 4-Heq), 2.56 (tt, J ϭ 2.4, J ϭ 12.8, 1 H,
5-Hax), 4.03 (m, 1 H, CHOH), 7.15Ϫ7.70 (m, 15 H, arom.CH)
ppm. 13C NMR (100 MHz, CDCl3): δ ϭ 136.4, 136.3, 128.3, 128.2,
128.0, 127.7 (arom. CH), 129.8 (arom. C), 67.7 (HCOH), 61.2
[C(SMe)2], 46.2 (2-CH2), 37.3, 37.2 (4,6-CH2), 18.6 (5-CHSi), 11.2,
10.9 (SMe) ppm.
[6]
[7]
[8] [8a]
J. E. Baldwin, J. Chem. Soc., Chem. Commun. 1976,
734Ϫ736; J. E. Baldwin, J. Cutting, W. Dupont, L. Krause, L.
Silberman, R. C. Thomas, J. Chem. Soc., Chem. Commun.
[8b]
1976, 736Ϫ741.
C. Thebtaranonth, Y. Thebtaranonth, Tet-
[8c]
rahedron 1990, 46, 1385Ϫ1489.
Res. 1993, 26, 476Ϫ482.
C. D. Johnson, Acc. Chem.
[9] [9a]
P. Auvray, P. Knochel, J. F. Normant, Tetrahedron Lett.
1985, 26, 4455Ϫ4458 and references cited therein. [9b] R. Grigg,
3-(Methylthio)-5-(triphenylsilyl)cyclohex-3-en-1-ol (22): IR (film):
ν˜ ϭ 3416 (OH), 1589 (CϭC), 1428, 1109 (CϪO), 1038, 847, 738,
702 cmϪ1 (Ph). 1H NMR (400 MHz, CDCl3): δ ϭ 1.72 (q, 2J ϭ
3J ϭ 3J ϭ 11.2, 1 H, 6-H), 2.11 (m, 1 H, 6-H), 2.17 (m, 1 H, 2-H),
2.46 (ddd, J ϭ 2.6, J ϭ 4.8, J ϭ 16.0, 1 H, 2-H), 2.56 (m, 1 H, 5-
H), 3.74 (m, 1 H, 1-H), 5.62 (t, 3J ϭ 4J ϭ 2.6, 1 H, 4-CH),
7.15Ϫ7.70 (m, 15 H, arom. CH) ppm. 13C NMR (100 MHz,
CDCl3): δ ϭ 136.4, 136.3, 128.3, 128.2, 128.0, 127.7 (arom. CH),
129.8 (arom. C), 129.8 (olef. C), 119.4 (olef. CH), 68.8 (HCOH),
39.4 (2-CH2), 33.7 (6-CH2), 25.7 (CHSi), 14.1 (SMe) ppm.
J. Kemp, J. F. Malone, S. Rajviroongit, A. Tangthongkum, Tet-
[9c]
rahedron 1988, 44, 5361Ϫ5374.
A. Padwa, B. H. Norman,
J. Org. Chem. 1990, 55, 4801Ϫ4807. [9d] A. Padwa, P. E. Yeske,
J. Org. Chem. 1991, 56, 6386Ϫ6390.
[10]
[11]
E. Schaumann, F. Tries, Synthesis 2002, 191Ϫ194.
J. E. Baldwin, R. M. Adlington, D. B. Russel, A. T. Russel,
Tetrahedron 1994, 50, 12015Ϫ12028.
D. Seebach, M. Kolb, B. Th. Gröbel, Chem. Ber. 1973, 106,
2277Ϫ2290; R. Bürstinghaus, D. Seebach, Chem. Ber. 1977,
110, 841Ϫ851.
[12]
[13] [13a]
E. J. Reist, I. Junga, B. R. Baker, J. Org. Chem. 1960, 25,
[13b]
1673Ϫ1674.
L. M. Harwood, G. Casy, J. Sherlock, Synth.
Commun. 1990, 20, 1287Ϫ1292.
A. G. Brook, J. M. Duff, D. G. Anderson, Canad. J. Chem.
1970, 48, 561Ϫ569.
Received October 10, 2002
[O02536]
Acknowledgments
We gratefully acknowledge support by the Fonds der Chemischen
Industrie.
[14]
1090
Eur. J. Org. Chem. 2003, 1085Ϫ1090